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Matthew T. Vaughan, Brian H. Tang, and Lance F. Bosart

Abstract

This study identifies high-impact severe weather events with poor predictive skill over the northeast United States using Storm Prediction Center (SPC) convective outlooks. The objectives are to build a climatology of high-impact, low predictive skill events between 1980 and 2013 and investigate the differences in the synoptic-scale environment and severe weather parameters between severe weather events with low predictive skill and high predictive skill. Event-centered composite analyses, performed using the National Centers for Environmental Prediction Climate Forecast System Reanalysis and the North American Regional Reanalysis, suggest low predictive skill events occur significantly more often in low-shear environments. Additionally, a plurality of low probability of detection (POD), high-impact events occurred in low-shear, high-CAPE environments. Statistical analysis of low-shear, high-CAPE environments suggests high downdraft CAPE (DCAPE) and relatively dry lower levels of the atmosphere are associated with widespread severe weather events. DCAPE and dry boundary layer air may contribute to severe wind gusts through strong negative buoyancy and enhanced evaporative cooling of descending saturated parcels.

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Matthew R. Kumjian, Alexander P. Khain, Nir Benmoshe, Eyal Ilotoviz, Alexander V. Ryzhkov, and Vaughan T. J. Phillips

Abstract

Polarimetric radar observations of deep convective storms frequently reveal columnar enhancements of differential reflectivity Z DR. Such “Z DR columns” can extend upward more than 3 km above the environmental 0°C level, indicative of supercooled liquid drops being lofted by the updraft. Previous observational and modeling studies of Z DR columns are reviewed. To address remaining questions, the Hebrew University Cloud Model, an advanced spectral bin microphysical model, is coupled with a polarimetric radar operator to simulate the formation and life cycle of Z DR columns in a deep convective continental storm. In doing so, the mechanisms by which Z DR columns are produced are clarified, including the formation of large raindrops in the updraft by recirculation of smaller raindrops formed aloft back into the updraft at low levels. The internal hydrometeor structure of Z DR columns is quantified, revealing the transition from supercooled liquid drops to freezing drops to hail with height in the Z DR column. The life cycle of Z DR columns from early formation, through growth to maturity, to demise is described, showing how hail falling out through the weakening or ascending updraft bubble dominates the reflectivity factor Z H, causing the death of the Z DR column and leaving behind its “ghost” of supercooled drops. In addition, the practical applications of Z DR columns and their evolution are explored. The height of the Z DR column is correlated with updraft strength, and the evolution of Z DR column height is correlated with increases in Z H and hail mass content at the ground after a lag of 10–15 min.

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